Field devices serving to capture and/or modify process variables are frequently used in process automation technology. Sensors, such as fill level measuring devices, flow meters, pressure and temperature measuring devices, pH-redox potential meters, conductivity meters, etc., are used for recording the respective process variables, such as fill level, flow, pressure, temperature, pH level, and conductivity. Actuators, such as, for example, valves or pumps, are used to influence process variables. Thus, the flow rate of a fluid in a pipeline section or a filling level in a container can be altered by means of actuators. Field devices, in general, refer to all devices which are process-oriented and which provide or edit process-relevant information. In addition to the aforementioned sensors and actuators, units that are directly connected to a field bus and used for communication with superordinate units, such as, for instance, remote I/Os, gateways, linking devices, and wireless adapters, are also generally referred to as field devices. The company group Endress+Hauser produces and distributes a large variety of such field devices.
Such field devices convert a physical or chemical measured variable to an analog output signal corresponding to the measured value and transmittable via a two-wire line. In order to also operate the field devices in safety-critical applications, these must meet certain specifications. For this purpose, the international standard, IEC/EN 61508, in particular, serves as the basis for the specification, design, and operation of a “Safety Instrumented System” (SIS for short), such as a field device. IEC/EN 61508 as a basic standard uses an approach based upon the risk assessment:
An assessment is made of the risk, based upon which the resulting required “Safety Integrity Level” (SIL for short) for components and systems with security functions is determined.
These SIL-rated components and systems should reduce the existing risk of a device, such as a field device, to an acceptable level, or “tolerable risk.” In this regard, errors which are uncertain of being detected, in particular, adversely affect the “tolerable risk.”
For increased safety, i.e., to minimize the “tolerable risk” of the field devices, the analog output signal that is output or is to be transmitted, and which is fed into the two-wire cable via two connection terminals, must be read back. In known field devices of the prior art, this is typically implemented via a shunt resistor, which is provided for measuring current in the field device. This shunt resistor is located in the field device behind all explosion protection and/or EMC (Electromagnetic Compatibility) measures. Fault currents caused by faulty explosion protection and/or EMC measures might, therefore, not be detected and contribute to errors which are uncertain of being detected in the determination of “tolerable risk.”